1. Field of the Invention
This invention is concerned with the manufacture of high quality lubricating oils, and in particular with lubricating oils derived from petroleum distillate fractions. It is especially directed to the preparation of low pour point lubricating oils having improved overnight cloud performance and color from crude oils of high wax content. This invention is specifically directed to catalytically dewaxing a waxy distillate lubricating oil with a ZSM-5 zeolite catalyst containing a noble metal hydrogenation component.
2. Description of the Prior Art
Refining suitable petroleum crude oils to obtain a variety of lubricating oils which function effectively in diverse environments has become a highly developed and complex art. Although the broad principles involved in refining are qualitatively understood, the art is encumbered by quantitative uncertainties which require considerable resort to empiricism in practical refining. Underlying these quantitative uncertainties is the complexity of the molecular constitution of lubricating oils. Because lubricating oils for the most part are based on petroleum fractions boiling above about 450.degree. F., the molecular weight of the hydrocarbon constituents is high and these constituents display almost all conceivable structure types. This complexity and its consequences are referred to in well-known treatises, such as, for example, in "Petroleum Refinery Engineering," by W. L. Nelson, McGraw-Hill Book Company, Inc., New York, N.Y., 1958 (Fourth Edition).
In general, the basic premise in lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant stock having a predetermined set of properties such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate that lubricant stock consists of a set of subtractive unit operations which removes the unwanted components. The most important of these unit operations include distillation, solvent refining, and dewaxing, which basically are physical separation processes in the sense that if all the separated fractions were recombined one would reconstitute the crude oil.
A refined lubricant stock may be used as such as a lubricant, or it may be blended with another refined lubricant stock having different properties. Or, the refined lubricant stock prior to use as a lubricant, may be compounded with one or more additives which function, for example, as antioxidants, extreme pressure additives, and V.I. improvers.
For the preparation of a high grade distillate lubricating oil stock, the current practice is to vacuum distill an atmospheric tower residuum from an appropriate crude oil as the first step. This step provides one or more raw stocks within the boiling range of about 450.degree. F. to 1050.degree. F. After preparation of a raw stock of suitable boiling range, it is extracted with a solvent, e.g., furfural, phenol, sulfalane, or chlorex, which is selective for aromatic hydrocarbons, and which removes undesirable components. The raffinate from solvent refining is then dewaxed, for example, by admixing with a solvent such as a blend of methyl ethyl ketone and toluene. The mixture is chilled to induce crystallization of the paraffin waxes which are then separated from the raffinate. Sufficient quantities of wax are removed to provide the desired pour point for the raffinate.
Other processes such as hydrofinishing or clay percolation may be used if needed to reduce the nitrogen and sulfur content or improve the color of the lubricating oil stock.
In recent years, catalytic techniques have become available for dewaxing of petroleum stocks. A process of that nature developed by Mobil Oil Corporation is described in U.S. Reissue Patent No. 28,398.
Many patents have now issued for catalytic dewaxing, specifically, catalytic dewaxing processes utilizing zeolites of the ZSM-5 type family. Such patents include U.S. Pat. Nos. 3,894,938; 3,755,138; 4,053,532; 3,956,102 and 4,247,388, assigned to Mobil Oil Corporation to name just a few. The entire contents of these patents are herein incorporated by reference.
Typically, the zeolite dewaxing catalyst is employed in intimate combination with one, or more, hydrogenation components such as tungsten, molybdenum, nickel, cobalt, or a noble metal such as platinum or palladium.
The dewaxing mechanism of catalytic hydrodewaxing is different from that of solvent dewaxing, resulting in some differences in chemical composition. Catalytically dewaxed products produce a haze on standing at 10.degree. F. above specification pour point for more than twelve hours--known as the Overnight Cloud (ONC) formation. The extent of this ONC formation is less severe with solvent dewaxed oils. Although such an ONC formation does not affect the product quality of catalytically dewaxed oils, it is beneficial to reduce the Overnight Cloud (ONC) formation, since in some areas of the marketplace any increase in ONC is considered undesirable.
Thus, although some of the catalytic dewaxing processes of the prior art do, indeed, result in the production of lubricating oils of enhanced properties, nevertheless, the instant invention is concerned with an improved process wherein pour point specifications can be met and the resulting product will have improved overnight cloud performance and a lighter ASTM color than has heretofore been possible with prior art catalytic dewaxing operations.